A clad steel plate is known, which ensures strength and toughness by means of a substrate sheet comprising any one of carbon steel and low-alloy steel, and ensures corrosion resistance by means of a cladding sheet comprising stainless steel.
A method for manufacturing a clad steel plate is disclosed in Nippon Kokan Giho (Nippon Kokan Technical Report), No. 106, Jan. 31, 1985, pages 12 and 13, which comprises the steps of:
placing a cladding sheet comprising stainless steel onto at least one of the surfaces of a substrate sheet comprising any one of carbon steel and low-alloy steel; welding together the peripheries of the substrate sheet and the cladding sheet to prepare a slab; heating the prepared slab to a temperature within the range of from 1,100 to 1,250.degree. C.; and hot-rolling the heated slab at a finishing temperature of at least 1,050.degree. C. to pressure-weld the substrate sheet and the cladding sheet together to obtain a clad steel plate comprising the substrate sheet and the cladding sheet (hereinafter referred to as the "prior art").
Although the above-mentioned prior art does not mention cooling of the clad steel plate obtained by hot-rolling, it is the usual practice to leave the clad steel plate to cool in the open air.
In the above-mentioned prior art, since the clad steel plate obtained by hot-rolling is left to cool in the open air, no improvement is available in strength of the substrate sheet. Strength of the substrate sheet is therefore low. There is no improvement either in corrosion resistance of the cladding sheet. Corrosion resistance of the cladding sheet is therefore inferior to that of a solution-treated stainless steel sheet.
In the prior art, the prepared slab may sometimes be heated to a temperature within the range of from 1,050.degree. to 1,300.degree. C., and the heated slab may sometimes be hot-rolled at a finishing temperature of at least 800.degree. C. In this case also, there occur problems similar to those described above.
With a view to solving the above-mentioned problems in the prior art to improve strength of the substrate sheet or to improve corrosion resistance of the cladding sheet, the following methods for manufacturing a clad steel plate are now under study:
(1) A method which comprises, when heating and then hot-rolling a slab comprising a substrate sheet and a cladding sheet to pressure-weld the substrate sheet and the cladding sheet together, applying a substantial accumulated reduction to the slab within the temperature region not allowing recrystallization of austenite in carbon steel or low-alloy steel composing the substrate sheet, i.e., applying the so-called controlled rolling, to improve strength of the substrate sheet (hereinafter referred to as the "controlled rolling type method").
In the controlled rolling type method mentioned above, in order to improve strength of the substrate sheet by the controlled rolling, it is necessary to apply a very intensive accumulated reduction to the slab within the temperature region not allowing recrystallization of austenite in carbon steel or low-alloy steel, particularly within a low-temperature region of up to 850.degree. C., and the finishing temperature of hot-rolling of the slab lowers accordingly. As a result, there takes place only insufficient reduction of the slab within a higher temperature region, as is required for ensuring pressure-welding of the substrate sheet and the cladding sheet. Furthermore, application of hot-rolling to the slab at a temperature of under 800.degree. C. causes precipitation of chromium carbide from the structure of the cladding sheet in the case where the cladding sheet is of austenitic stainless steel, and precipitation of .sigma.-phase from the structure of the cladding sheet in the case where the cladding sheet is of dual-phase stainless steel, thus resulting in deterioration of corrosion resistance of the cladding sheet. This deterioration of corrosion resistance of the cladding sheet tends to be accelerated even if the slab has been previously heated before the controlled rolling to a temperature sufficient to cause dissolution of chromium carbide, since the clad steel plate is left to cool in the open air after the controlled rolling and chromium carbide may be reprecipitated.
(2) Another method which comprises leaving a clad steel plate obtained by hot-rolling a slab comprising a substrate sheet and a cladding sheet to cool in the air, and then reheating the clad steel plate to subject same to a heat treatment for the main purpose of a solution treatment of the cladding sheet, so as to give desirable properties to the cladding sheet and the substrate sheet (hereinafter referred to as the "solution treatment type method").
In the solution treatment type method mentioned above, when selecting a reheating temperature for the heat treatment for the main purpose of the solution treatment of the cladding sheet with a view to improving corrosion resistance thereof, a very high reheating temperature such as 1,010.degree. C. or higher would be required in order to dissolve chromium carbide into the structure of stainless steel composing the cladding sheet in the form of chromium. When heated to such a high temperature, the austenite grains in the structure of carbon steel or low-alloy steel composing the substrate sheet are abnormally coarsened, thus resulting in deterioration of toughness of the substrate sheet. On the other hand, when a reheating temperature for the heat treatment is selected mainly for improving strength and toughness of the substrate sheet, the selected temperature would be lower than the temperature for dissolving chromium carbide into the structure of the cladding sheet in the form of chromium, thus resulting in insufficient dissolution of chromium carbide, and hence in deterioration of corrosion resistance of the cladding sheet.
After all, in the solution treatment type method, it is inevitable to find out a point of compromise by selecting, as the reheating temperature for the heat treatment, an intermediate temperature between the reheating temperature for the heat treatment for improving corrosion resistance of the cladding sheet and the reheating temperature for the heat treatment for improving strength and toughness of the substrate sheet, and such a selection is in consequence unsatisfactory for improvement of corrosion resistance of the cladding sheet and improvement of strength and toughness of the substrate sheet.
A method for manufacturing a clad steel plate is described in the earlier Japanese Patent Application No. 72,972/84 filed on Apr. 13, 1984, which comprises the steps of:
placing a first cladding sheet comprising any one of austenitic stainless steel and dual-phase stainless steel onto a first substrate sheet comprising any one of carbon steel and low-alloy steel; applying a peeloff material onto the upper surface of the first cladding sheet; placing a second cladding sheet comprising any one of austenitic stainless steel and dual-phase stainless steel onto the first cladding sheet through the peeloff material therebetween; placing a second substrate sheet comprising any one of carbon steel and low-alloy steel onto the second cladding sheet; welding together the peripheries of the first substrate sheet and the second substrate sheet through spacers to prepare a slab; heating the prepared slab to a temperature of at least 1,050.degree. C.; hot-rolling the heated slab within a temperature region of from 850.degree. to 950.degree. C. at an accumulated reduction rate of from 30 to under 80% and at a finishing temperature of at least 850.degree. C. to pressure-weld the first substrate sheet and the first cladding sheet together, and simultaneously pressure-weld the second substrate sheet and the second cladding sheet together, to obtain simultaneously a first clad steel plate comprising the first substrate sheet and the first cladding sheet, and a second clad steel plate comprising the second substrate sheet and the second cladding sheet; cooling simultaneously the first clad steel plate and the second clad steel plate at a cooling rate of from 2.degree. to 30.degree. C. per second until the temperatures of the first clad steel plate and the second clad steel plate fall within the range of from 45.degree. to 650.degree. C., and leaving the first clad steel plate and the second clad steel plate to cool in the open air; and separating the first clad steel plate and the second clad steel plate from an interface applied with the peeloff material (hereinafter referred to as the "earlier application art").
In the above-mentioned earlier application art, cooling of the first clad steel plate and the second clad steel plate obtained by hot-rolling is carried out at a cooling rate of from 2.degree. to 30.degree. C. per second until the temperatures of the first clad steel plate and the second clad steel plate fall within a rather high temperature range of from 450.degree. to 650.degree. C., thus resulting in only a limited improvement in strength of the first substrate sheet and the second substrate sheet.
Although the above-mentioned earlier application art does not define the upper limit of the heating temperature of the prepared slab, the upper limit of the heating temperature of the slab is practically 1,300.degree. C. The heated slab may sometimes be hot-rolled at a finishing temperature of at least 800.degree. C. In this case, there also occur problems similar to those mentioned above.
Under such circumstances, there is a demand for the development of a method for manufacturing a high-strength clad steel plate excellent in corrosion resistance, in which a cladding sheet has a high corrosion resistance and a substrate sheet has a high strength, but a method for manufacturing a clad steel plate provided with such properties has not as yet been proposed.